Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of carbon monoxide and hydrogen (synthesis gas) with carbon compounds containing olefinic unsaturation. The reaction is typically performed in the presence of a carbonylation catalyst and results in the formation of compounds, for example, aldehydes, which have one or more carbon atoms in their molecular structure than the starting olefinic feedstock. In commercial operation, the aldehyde product is typically used as an intermediate which is converted by hydrogenation to an alcohol or by aldolization and hydrogenation to a higher alcohol. The aldol-hydrogenation route is used primarily for the manufacture of 2-ethylhexanol from propylene that is converted to n-butyraldehyde. The crude product of the hydroformylation reaction will contain a homogeneous catalyst, aldehydes, alcohols, unreacted olefin feed, synthesis gas and by-products, and in most cases, ligands. Homogeneous catalysts usable with the present invention are soluble in hydrocarbons or oils.
A variety of transition metal complexes catalyze the hydroformylation reaction, but only cobalt and rhodium carbonyl complexes are used in commercial plants. The reaction is highly exothermic; the heat release is approximately 125 kJ/mol (30 kcal/mol). The position of the formyl group in the aldehyde product depends upon the olefin type, the catalyst, the solvent, and the reaction conditions such as temperature and pressure. With several catalysts and reaction conditions, a predominantly straight chain product can be formed from a linear olefin feed.
Much research in the past twenty-five years has been directed to improving reaction selectivity to the linear product. The linear aldehydes which can be formed with rhodium catalyst complexes are intermediates for formulating biodegradable detergents, plasticizers, specialty polymers, etc. It has been found that with an unmodified cobalt catalyst (i.e., a catalyst having no ligand), the yield of a straight chain product is favored by very high CO partial pressures. Introduction of an organophosphine ligand to form an oil-soluble complex, e.g., Co.sub.2 (CO).sub.6 [P(n-C.sub.4 H.sub.9).sub.3 ].sub.2, can significantly improve the selectivity to the straight-chain alcohol under high pressure conditions. Rhodium catalysts containing selected complexing ligands, e.g., tertiary phosphines, can result in the predominant formation of the normal isomer with lower CO and H.sub.2 partial pressures. In the most widely used commercial process for formation of linear aldehydes using a ligand-modified rhodium-catalyst, the reactor contains the rhodium complex catalyst, excess triphenylphosphine, CO, H.sub.2 and a mixture of product aldehydes and condensation by-products. The product aldehyde, which is low in molecular weight and relatively volatile, may be recovered from the mixture by volatilization directly from the reactor or by distillation in a subsequent step. The catalyst either remains in or is recycled to the reactor. However, the complex catalyst and triphenylphosphine ligand are slowly deactivated and eventually the spent catalyst is removed for recovery of rhodium and reconversion to the active catalyst. This process, although effective for lower molecular weight aldehyde production, is not favored for higher molecular weight aldehydes which are higher boiling. Distillation temperatures needed for high boiling aldehyde recovery can cause catalyst deactivation to be accelerated.
Unfortunately, the low pressure rhodium catalyst systems that have been used commercially for the hydroformylation of propylene feedstocks to produce butyraldehyde cannot normally be used in industrial processes to make higher aldehydes because conventional separation technologies cannot remove the product aldehydes from the homogeneous rhodium catalyst complexes without significant destruction or rhodium loss. It is an object of this invention to overcome this limitation imposed by conventional separation technologies. Overcoming these limitations is particularly important for rhodium-based catalysts because of the high cost of rhodium metal.
Homogeneous ligated rhodium catalyst complexes can be formed with a variety of organophosphines in the presence of carbon monoxide and hydrogen. A typical homogeneous rhodium catalyst complex is formed with triphenylphosphine ligands in the presence of carbon monoxide and hydrogen. Under optimal conditions homogeneous rhodium catalyst complexes formed with triphenylphosphine, CO and H.sub.2 have been reported to convert linear olefins into linear aldehyde products with less than approximately 1% isomerization under the most optimal conditions and approximately 5% isomerization under more practical conditions. The rhodium bonds to the triphenylphosphine ligand through a phosphorous atom. A schematic diagram of the triphenylphosphine ligand is set forth below: ##STR1## A large number of complexes are formed between rhodium, triphenylphosphine, hydrogen, and carbon monoxide, because they form loosely bound molecular species which are involved in multiple equilibria as they dissociate and recombine with ligands in solution. Some of the reaction pathways in these multiple equilibria are set forth below: ##STR2## At least one of the complexes in this multiple equilibria can be a very active catalyst for the hydroformylation reaction which converts linear olefins into the next higher carbon number linear aldehydes by the addition of carbon monoxide and hydrogen. In addition, the catalyst causes some of the product aldehyde to react to dimer and trimer condensation products. The isomerization activity of the catalyst is extremely undesirable in applications designed to produce long chain linear aldehydes. Linear aldehydes containing between 12 to 15 carbon atoms are readily hydrogenated to linear alcohols which are premium products for formulating biodegradable liquid detergents.
Separation of several types of homogeneous rhodium/organophosphine ligand complexes from high boiling aldehyde products has been previously attempted using conventional separation techniques such as distillation, and liquid/liquid extraction. Even in a carbon monoxide and hydrogen atmosphere, rhodium/organophosphine ligand complexes are unstable at the elevated temperatures used for vacuum distillation of long chain aldehydes. Liquid/liquid separations based on the phase behavior of water soluble catalysts have also been attempted. These separations have been tried in cases with high boiling reaction products where the olefin feed and reaction products are not very soluble in water. In such cases, it is often advantageous to add a surfactant to the aqueous medium to enhance phase contacting so as to improve catalytic rate and selectivity to the desired products. This type of process is called "Phase Transfer Catalysis." However, when the surfactant is added, some carry-over of the noble metal into the organic phase at the conclusion of the process often results. This and other types of catalyst loss have made it impractical to make high boiling products using aqueous catalyst in processes where the products are decanted from an aqueous catalyst solution.
A variety of membrane separation processes have also been tested for separating high boiling point products from an oil soluble catalyst complex. Attempts have been made to create large catalyst complexes which could be separated by ultrafiltration. In one case, high molecular weight phosphine ligands were used to form a homogeneous catalyst complex. High molecular weight polymeric phosphine ligands are synthesized by reacting polyvinylchloride, polychloroprene or brominated polystyrene with lithium diphenylphosphide at 20.degree. C. to 25.degree. C. These homogeneous catalysts containing bulky ligands are thought to be more easily separated from the reaction products by ultrafiltration. See Imyanitov et al., All-Union Scientific Research Institute of Petrochemical Processes, Neftekhimiya, 32, No. 3:200-7 (May-June 1992). The process described herein uses a smaller catalyst complex which is not attached to a polymeric backbone.
It has also been known to use membranes to separate water-soluble catalysts from an aqueous solution. An example is set forth in European Patent No. 0263953, published on Aug. 29, 1986 (assigned to Ruhrchemie Aktiengesellschaft), which discloses a process for separating rhodium complex compounds, which contain water-soluble organic phosphines as ligands, from aqueous solutions in which excess phosphine ligand and, if necessary, other components are also dissolved, is characterized by the fact that the aqueous solution is subjected to a membrane separation process. According to this process, volatile organic substances are separated from the solution prior to conducting the membrane separation process. A typical membrane for use in this process is a cellulose acetate membrane. This process will not work with the types of oil-soluble catalysts used in the present invention.
Another patent which utilizes cellulose acetate, silicone rubber, polyolefin or polyamide membranes in the separation of catalysts from high boiling by-products of the hydroformylation reaction is Great Britain Patent No. 1312076, granted on May 15, 1970. According to this patent the aldehydes produced during the hydroformylation process are continuously withdrawn as an overhead vapor stream. The liquid stream containing the heavy by-products with the catalyst is passed over a membrane wherein approximately 78-94.3% of the catalyst is retained and the heavy by-products permeated. This is an unacceptably low level of catalyst retention which is overcome by the process of the present invention. Also in the present invention, the aldehyde product is contacted with the membrane rather than withdrawn as an overhead vapor stream.
In like manner, Great Britain Patent No. 1432561, granted on Mar. 27, 1972, (assigned to Imperial Chemical Industries LTD.) discloses a process for the hydroformylation of olefins which comprises reacting an olefin at elevated temperature and pressure with CO and H.sub.2 in the presence of a compound of a group VIII metal and a biphyllic ligand of a trivalent P, As or Sb to give a crude liquid hydroformylation product containing an aldehyde and/or an alcohol, separating the aldehyde and/or alcohol from the crude product and leaving a liquid, bringing the liquid after separation of the Group VIII metal compound and free from aldehyde and alcohol under reverse osmosis conditions into contact with one side of a silicone rubber semi-permeable membrane in which the polymer chains have been at least partly cross-linked by gamma radiation, whereby the liquid retained by the membrane contains a higher concentration of Group VIII metal compounds and/or biphyllic ligand than the original liquid. Reverse osmosis membranes are different in composition and separate by a different mechanism than the membranes used in the present invention.
In an article by Gosser et al., entitled "Reverse Osmosis in Homogeneous Catalysis," Journal of Molecular Catalysis, Vol. 2 (1977), pp. 253-263, a selectively permeable polyimide membrane was used to separate soluble transition metal complexes from reaction mixtures by reverse osmosis. For example, separation of cobalt and rhodium complexes from hydroformylation products of 1-pentene. That is, a solution of 0.50 g of RhH(CO)(PPh.sub.3).sub.3 in 40 ml of benzene and 10 ml of 1-pentene was stirred at 50.degree. C. with a CO/H.sub.2 mixture at ca. 4 atm pressure until no further pressure drop occurred. The pentene was completely converted to aldehydes according to proton nmr analysis. The solution was permeated through a polyimide membrane under 68 atm nitrogen pressure. The permeate (4.5 g passed in 2 min.) showed only 9% of the original rhodium concentration by X-ray fluorescence. The permeation rate of rhodium as set forth above, i.e., 9%, is considered unacceptable. The rhodium catalyst should be retained in an amount of greater than 99.5% to be a commercially feasible process. The technique employed does not use the dense polymer membranes nor the operating conditions used in the present invention.
Another example of the use of membranes to separate metal catalysts from hydroformylation products is set forth in Dutch Patent No. 8700881, published on Nov. 1, 1988. The method disclosed therein relates to one which improves the efficiency of membrane separation of hydroformylation products from expensive organometallic catalyst containing reaction mixtures. In Dutch Patent No. 8700881 a polydimethylsiloxane membrane having a thickness of 7 microns applied to a Teflon.RTM. support was used in the separation of a reaction mixture containing C.sub.9 -C.sub.15 alcohols, a homogeneous catalyst system comprising an organometallic complex of a transition metal from Group VIII or VIIa or Va of the Periodic Table, e.g., a tricarbonyl-(triphenylphosphine) cobalt catalyst, and 40% low-viscosity lubricating oil (an antiswelling or de-swelling agent). At a flow of 133 kg/m.sup.2 -day, the cobalt contents in the feed, retentate, and permeate were 600, 910, and 18 ppm, versus 840, 1930, and 160 ppm, respectively, for a mixture without the deswelling agent. The ligands disclosed in Dutch Patent No. 8700881 are all organic soluble ligands, e.g., triphenylphosphine, tri-n-alkylphosphine or acetyl acetonate. Critical to the process of Dutch Patent No. 8700881 is the addition of a de-swelling agent to the reaction mixture which assists in the separation of the products from the reaction mixture. The present invention operates without the use of a de-swelling agent.
The present inventors have been examining whether rhodium separation from hydroformylation products can be performed with a membrane when the catalyst complexes are formed using hydrocarbon or oil soluble phosphine ligands in the presence of an atmospheric mixture of CO and H.sub.2. They have discovered that alkylated phosphine ligands together with dense nonpolar polymeric membranes are capable of substantially retarding the rhodium loss during the separation of the rhodium catalyst from the hydroformylation reaction products. It was also discovered that triphenylphosphine ligands used in conjunction with a dense polymeric, nonpolar membrane also substantially retards rhodium catalyst loss, although not as well as alkylated phosphines. Optimum operating conditions for the present invention involve performing the separations in an atmosphere of CO and H.sub.2 each with partial pressures less than one atmosphere.
The present invention also provides many additional advantages which shall become apparent as described below.